Obesity and its associated medical complications represent a major health problem in black women. Evaluating the prevalence and underlying pathogenesis of obesity in black women requires accurate estimates of body fat and the metabolically active tissue mass. At present body weight is subdivided into fat and fat-free fractions through models based on an assumed density, K concentration, or hydration of the fat-free body mass (FFM). Although hydrodensitometry, whole body counting for K, and total body water methods are considered research level techniques, other methods calibrated against them are widely used in clinical practice or in field surveys. The evaluation of human body composition thus relies to a very large extent on the two- compartment models which are based on or validated n relatively few white male cadavers. Until recently , pushing beyond these few cadaver studies in vivo was limited by the unavailability of he required methodology. Major technological advances now, however, provide the opportunity to recalibrate two-compartment methodology and to explore previously unmeasurable body compartments. In the initial phase of our funded study we combined unique resources (hydrodensitometry, dual photon absorptiometry (DPA), radioactive isotopes, neutron inelastic scattering, and delayed and prompt gamma neutron activation) to estimate body density, bone ash, H2O, exchangeable Na, and total body K, Na, Cl, N, P, Ca, and C. By applying these techniques we: 1) developed or markedly improved three-multicompartment body composition models, 2) evaluated four methods of estimating FFM density in vivo, 3) validated the use of DPA in deriving bone mineral, appendicular skeletal muscle, and total body fat, to demonstrate black-white differences in matched (age, weight, height) women for bone mineral, total body K, and FFM density; and 5) observed a relative increase in hydration and lowering of FFM density in obese black women compared to matched lean controls. The aims of this renewal are to 1) expand our subject pool (n-7) to provide definitive coefficients for two-compartment models in women as a function of age, ethnicity, and body fat; 2) model resting metabolic rate in relation to extensive body composition measurements in this diverse group of subjects; and 3) critically evaluate body composition methodology and models in tracking compartmental changes in obese women losing weight on a hypocaloric diet over three months.